This may seem obvious, but for me it was not. I wanted to put images on SharePoint pages, but have the image be responsive to the web-part zone. I found this was actually really easy to do with a script editor web-part and some help from the w3schools.com website.
This code is all you need:
<meta name=”viewport” content=”width=device-width, initial-scale=1″>
<p>If you want to restrict a responsive image to a maximum size, use the max-width property, with a pixel value of your choice.</p>
<p>Resize the browser window to see the effect.</p>
<img src=”img_nature.jpg” alt=”Nature” class=”responsive” width=”600″ height=”400″>